Peptidyl-tRNA dissociates from ribosomes of E. coli during protein biosynthesis. The process can be altered by pharmacological, physiological and genetic perturbations to the bacterial cell. We hypothesize that peptidyl-tRNA dissociates preferentially after peptide is transferred to an inappropriate aminoacyl-tRNA in the ribosome A site, as the result of a postulated ribosomal editor. We propose to test this idea by examining the protein synthesis by mutant bacterial cells or extracts programmed with a minimum number of mRNAs and the response of this synthesis to various perturbations. The peptides of the peptidyl-tRNA that accumulates in the mutant cells or extracts at high temperatures will be analysed for length and sequence.